Load-time Curves Research Articles

A quantitative hot tearing criterion for aluminum alloys

In this study, a new hot tearing criterion was proposed to quantitatively predict open and segregated hot tears in aluminum alloys. In the suggested model, the displacement of grains at a grain boundary near the solidus temperature was considered as the main reason for crack formation. In this model, it is assumed that when the distance between two neighbor grains exceeds the distance between those grains at the coherency temperature, the enriched liquid can penetrate to the root of the dendrites and form segregated and open hot tears. The main parameters of the model are microstructure (grain size and number of grain boundaries), critical strain, applied strain, and width of generated cracks. The index was verified by the solidifying shell tensile (SST) test of an Al alloy containing 0.95 wt% Cu and 0.12 wt% Fe. Also, the critical strain for crack formation in the test was calculated by using accumulated strain theory and load-time curves. The results revealed that the proposed model can suitably predict the number of open and segregated hot tears generated in the SST test samples.

Simulation and Experimental Study on Stress Relaxation Response of Polycrystalline γ-TiAl Alloy under Nanoindentation Based on Molecular Dynamics.

This study employed nano-indentation technology, molecular dynamics simulation, and experimental investigation to examine the stress relaxation behaviour of a polycrystalline γ-TiAl alloy. The simulation enabled the generation of a load-time curve, the visualisation of internal defect evolution, and the mapping of stress distribution across each grain during the stress relaxation stage. The findings indicate that the load remains stable following an initial decline, thereby elucidating the underlying mechanism of load change during stress relaxation. Furthermore, a nano-indentation test was conducted on the alloy, providing insight into the load variation and stress relaxation behaviour under different loading conditions. By comparing the simulation and experimental results, this study aims to guide the theoretical research and practical application of γ-TiAl alloys.

Valacyclovir or valganciclovir for cytomegalovirus prophylaxis: A randomized controlled trial in adult and pediatric kidney transplant recipients

BackgroundValganciclovir (valG), a cytomegalovirus (CMV) prophylactic agent, has dose-limiting side effects. The tolerability and effectiveness of valacyclovir (valA) as CMV prophylaxis is unknown. MethodsWe conducted a randomized, open-label, single-center trial of valA versus valG for all posttransplant CMV prophylaxis in adult and pediatric kidney recipients. Participants were randomly assigned to receive valA or valG. Primary endpoints were the incidence of CMV viremia and side-effect related drug reduction with secondary assessment of incidence of EBV viremia. ResultsOf the 137 sequential kidney transplant recipients enrolled, 26 % were positive and negative for CMV antibody in donor and recipient respectively. The incidence of CMV viremia (4 of 71 [6 %]; 8 of 67 [12 %] P = 0.23), time to viremia (P = 0.16) and area under CMV viral load time curve (P = 0.19) were not significantly different. ValG participants were significantly more likely to require side-effect related dose reduction (15/71 [21 %] versus 1/66 [2 %] P = 0.0003). Leukopenia was the most common reason for valG dose reduction and granulocyte-colony stimulating factor was utilized for leukopenia recovery more frequently (25 % in valG vs 5 % in valA: P = 0.0007). Incidence of EBV viremia was not significantly different. ConclusionsValA has significantly less dose-limiting side effects than valG. In our study population, a significant increase in CMV viremia was not observed, in adults and children after kidney transplant, compared to valG. Trial Registration NumberNCT01329185

Experimental Study on Plastic Deformation of EH690 Steel with the V-Notch during Falling Weight Impact Based on DIC

In this study, a falling weight impact test was conducted on EH690 steel specimens with V-notches using Digital Imaging Correlation (DIC). In conjunction with scanning electron microscopy (SEM), the plastic deformation and crack initiation processes were examined at the notch of the specimen under different impact energies (90 J, 120 J, 135 J and 150 J). ABAQUS was used to simulate the plastic deformation of an EH690 specimen. The results show that the strain at the notch tip experienced some elasticity and yielding as the load increased under different impact energies. The load remains unchanged or decreases slightly when a plastic hinge forms at the tip of the notch. According to the microscopic images, there are three areas on the fracture surface: a fiber area, a radiation area, and a shear lip area. With increasing deformation, a crack source forms in the middle of the V-shaped notch and propagates to the inside and outside surfaces of the sample. Cracks are primarily caused by ductile tears. The use of DIC to analyze the surface strain of EH690 steel specimens was verified by comparing DIC with finite element analysis. Both curves have the same trend and the maximum error in the load-time curve is 9.42%, the maximum error in the displacement–time curve is 5.61%, and the maximum error in the strain-time curve is 10.68%.

Investigation of the impact of heating and liquid nitrogen (LN2) cooling on the mechanical and acoustic emission (AE) properties of coal

Liquid nitrogen has emerged as a promising fracturing medium for unconventional natural gas extraction, particularly in the context of coalbed methane extraction, generating significant interest in recent times. In this paper, we analyze the acoustic emission (AE) characteristics of coal samples treated with liquid nitrogen under three-point bending load conditions using indoor experiments and field investigations. We investigate the impact of liquid nitrogen on coal samples by analyzing AE data. The mechanical properties of coal samples with varying initial temperatures were tested under three-point bending load conditions after treatment with liquid nitrogen. The load-displacement curve was analyzed to study the mechanical properties of the coal samples after treatment with liquid nitrogen. Through indoor three-point bending experiments, data analysis, and knowledge of mechanics, we systematically investigated the changes in mechanical properties caused by liquid nitrogen treatment under different conditions. The results demonstrate that the application of liquid nitrogen has significant impacts on the mechanical and AE characteristics of coal samples. The load-displacement curve indicated that the mechanical properties of the coal samples changed after treatment with liquid nitrogen. The AE experiment revealed regular changes in vibration and energy parameters of coal samples after liquid nitrogen treatment under different conditions. We further investigated the changes in mechanical properties and deformation characteristics of coal samples after undergoing liquid nitrogen freezing and thawing in differing environments. Graphs such as load-displacement curves, load-time curves, cumulative vibration count-energy count graphs, and RA-AF density distribution diagrams visually demonstrated the changes in mechanical properties and deformation characteristics. Through an integrated approach of indoor experiments, data analysis, and knowledge of mechanics, our study provides a better understanding of the behavior of coal samples under different conditions. This understanding can contribute to the development of safer and more efficient methods for extracting coalbed methane.

Study on damage and fracture characteristics of wood based on acoustic emission b-value and seismic magnitude difference entropy

To assess the damage and fracture behavior of wood under load, a wood damage assessment method was proposed based on acoustic emission (AE) b-values and seismic magnitude difference entropy. First, AE signals from Pinus sylvestris var. mongolica (softwood) and Zelkova schneideriana (hardwood) specimens were collected separately at a sampling frequency of 500 kHz in a three-point bending test. Then, 52 dB was taken as the threshold of the AE event, and the b-value and seismic magnitude difference entropy were calculated at 4-s intervals. Finally, by comparing with the load‒time curve, the b-value and seismic magnitude difference entropy were used to evaluate the damage fracture degree. The results showed that an increase in the b-value indicates the accumulation of strain energy, and vice versa, corresponding to the concentrated release of strain energy. At the same time, the test process can be divided into three stages—elastic, elastic‒plastic and plastic—based on the level of the seismic magnitude difference entropy.

Experimental and numerical studies on low-velocity impact of laminated C/SiC structures

Laminated C/SiC(carbon fiber reinforced silicon carbide composites) has the complicated nature of composite failure which include cracks for void defects at the meso and micro scales, complex cracks propagation of tunnel cracks, matrix cracks, delaminations and fiber cracks, orthotropic, and pseudoplastic behavior. It has technical challenge to solve all of them for complicated-mechanics problems by using multi-scale methodology. This paper focused tackling it by a model that lies between macro and meso scales, and proposed a new conceptual 3D FE-PDM(Finite Element - Progressive Damage Method). 3D FE-PDM was validated by the experiments of low-velocity impact of 2D C/SiC and 3D needled C/SiC. Damage initiation and evolution were controlled by strain in order to preventing stress chaos which could occur as cracks leading local stress relief. The multi-linear constitutive relations were adopted in orthotropic directions. The coupled effects of stresses between in-plane and out-of-plane were computed by adding damage coefficients into stiffness matrix. Cohesive Zone Method was used for delamination. The PDM approach was introduced for the 3D elements for intralaminar and cohesive elements for the interfaces. A parametric inversion methodology was put forward to determine the model parameters iteratively by comparing simulation results with the load-time curves, damage CT details of the experiments. Validation implies that, with small number of parameters, 3D FE-PDM is able to predict damage initiation of each phase knee, damage evolution, and load–displacement curve with good convergence for the low-velocity impact case. 2D woven and 3D needled non-woven C/SiC both have impact energy threshold and different damage evolution types.

Multi joint short term resistance training strength zone evaluated on maximal strength, maximal ısometric strength, peak isom reach, maximal isometric endurance and muscular local endurance in resistance trained men

To resistance trained men multi joint short term strength zone periodization for strength training non-periodic progression producted by performing peak isom reach periodize model on maximal strength and maximal isometric strength for peak isom reach periodize model for strength development on isometric strength. Other maximal isometric endurance and muscular local endurance were provided on load-time curve intensity in muscular isom change load intensity. In this case, determination on strength zone periodization was performed on 1-5RM strength zone one session set-up and maximal isometric strength on load-time curve at peak isom reach development. The peak isom reach for periodic strength sessions were conducted on strength/speed at 80% of 1RM (2.5 s) and maximal strength/force at 90% of 1RM (4 s) on maximal isometric strength. Indeed, multi joint strength zones developed the maximal squat (ES=0.55; p=0.000; t=-4.837), bench press (ES=0.46; p=0.000; t=7.386) and deadlift (ES=0.50; p=0.000; t=-4.837) in resistance trained men. This study were noted to peak isom reach periodize model performance developed on multi joint load repetition may be use for maximize performance in continuum repetition zone..

Acoustic Emission Characteristics and Damage Evolution Analysis of Sandstone under Three-Point Bending Test

In order to study the prevention of roadway roof bending, sinking, and breaking and the prevention of dynamic disasters such as the appearance of rock bursts, three-point bending experiments of sandstone under different spans were carried out. By using stress loading system and acoustic emission technology, the acoustic emission characteristic parameters of the sandstone fracture process were analyzed, the precursory information of rock bending fracture was explored, and the evolution law of sandstone damage based on acoustic emission characteristics was obtained. The results showed that according to the variation of acoustic emission ringing count, the load-time curve was divided into four typical stages: the first stage showed an overall increasing trend; the second stage showed an obvious increasing trend; the third stage showed obvious accelerated growth and the acoustic emission ringing count reached the maximum at the moment of rupture. In the fourth stage, the amplitude and frequency of the ringing count are large and high. With the increase in span, the cumulative ringing counts of AE decreased, and the rate of change gradually decreased. The fracture process of three-point bending sandstone can reflect the precursor information of rock fracture from time domain, frequency domain, and R value (ratio of cumulative acoustic emission ringing count to cumulative energy count). In time domain, the evolution characteristics of AE cumulative ringing count stage II can be used to predict the three-point bending fracture of sandstone. The peak frequency shows a linear increase after a spike and then an accelerated increase to a rupture, and the boundary point between the spike and the linear and nonlinear can be obtained. The decrease in R value can indicate that the main crack is growing in the specimen. According to the damage rate, the characteristics of the damage variables can be divided into five stages: stationary and low, gradually increasing, gradually decreasing, main crack coalescence, and complete fracture. With the increase in span, the fracture damage shows a decreasing trend. The residual damage fluctuates between 0.38 and 0.40 due to the difference in crack propagation trajectory. It has theoretical research value for revealing the internal mechanism of rock bending and fracture and has engineering guiding significance for mine pressure and rock strata control and dynamic disaster prevention.

Study on crack propagation in layered rock under the three-point bending conditions

The pattern of layered rock crack propagation in mining and geotechnical engineering directly influences the production efficient of coal mines and the safety of workers. To reveal the characteristics of crack propagation in layered rock, the three-point bending experiment and numerical investigation were carried out to study the crack propagation process of single sandstone samples, sandstone-sandstone-sandstone combined samples, and sandstone-mudstone-sandstone combined samples. Using digital speckle correlation methods and acoustic emission techniques, the crack propagation process of the specimens was examined. The results show that the main factors affecting crack propagation include rock layer strength, bedding plane strength, and the position of pre-existing fractures. The presence of the bedding plane hinders the propagation of cracks, and when the bedding plane’s strength decreases, cracks that originated from the bedding plane spread horizontally along it. The weakening of the intermediate rock layer strength of the combined samples can lead to the initiation of microcracks within it earlier than macroscopic cracks, allowing many options for the initiation and propagation of following cracks. Meanwhile, the load-time curves of the combined samples during the experiment displays a double-peak characteristic. Furthermore, the presence of pre-existing fractures inside the layered rock might result in the unpredictability of the crack starting point and subsequently influence the crack propagation course.

Fracture characteristics analysis of FRP-reinforced concrete based on acoustic emission

To study the acoustic emission (AE) characteristics of fibre-reinforced polymer (FRP)-reinforced concrete during the fracture process, three-point bending tests of FRP-reinforced concrete beams with cracks were carried out. The AE characteristics of FRP-reinforced concrete beams with different bonding lengths and different bonding layers were analysed based on the AE parameters, and the effect of the fracture parameters was studied. The test results reveal that the crack initiation load and peak load of single-layer FRP-reinforced concrete beams first increase and then decrease with the increase of bonding length. The failure mode of multi-layer FRP-reinforced concrete beams is a kind of dangerous brittle failure, which easily forms the ‘super-reinforcement failure’ mode. The ductility of single-layer FRP-reinforced concrete beams with a bonding length of 25 cm is optimal, which can effectively delay crack propagation. By combining the load–time curves, AE ringing count–time curves and AE energy–time curves, the time points of crack initiation load and instability load can be accurately determined. The analysis of RA–AF (ratio of ringing count to duration–ratio of rise time to maximum amplitude) and clearance factor index can be used to evaluate the damage state of FRP-reinforced concrete beams.

Experimental Study on Deformation Characteristics of Gangue Backfill Zone under the Condition of Natural Water in Deep Mines

To address the problem of surface subsidence caused by the compression of filling gangue in deep mines, a layered compaction test was designed based on the zonation of the failure of the overburden in the goaf and layered property of the filling gangue. The deformation characteristics of filling gangue in natural and water-bearing states were obtained. The deformation of filling gangue during the 0~100 kN loading stage was an approximately positive S-type, which reflects the relative “advancement” in terms of deformation. The filling gangue deformation in the 100~500 kN loading stage was an approximately inverted S-type, which reflects the relative “lag” in terms of deformation. In a natural state, the load-time curves of the dead load stage were consistent. Under a water-bearing condition, the load-time curve for the dead load stage had apparent “step” characteristics and presented a special phenomenon of displacement rebound. Under gradient loading, the strain showed an exponential growth model, and energy dissipation showed a logarithmic growth model. Under a natural state, the energy dissipation showed consistently increasing distribution patterns, while the energy dissipation showed a normal distribution model under the water-bearing state.

Study on the Testing Method of Relaxation Modulus under Spherical Indenter Loading

Viscoelastic materials are widely used in various fields. In order to better use viscoelastic materials in various working conditions, it is necessary to master the material properties of viscoelastic materials. Relaxation modulus is a characterization method of viscoelastic material properties. In this paper, by analyzing that viscoelastic materials are time-domain related parameters, the load-time curves are obtained by using the spherical indentation instrument to carry out the ramp loading indentation test at different speeds. According to the law of ramp strain history loading, the data at different velocities are summarized and a method for judging the linear viscoelastic boundary of viscoelastic materials in spherical indentation tests is obtained. Aggregate the load-time data at different speeds to obtain a set of valid data for a longer period of time. A method of measuring the relaxation modulus using the ramp-constant strain history is proposed. Compared with the ramp loading test method, it is simple and easy to operate, and it can make up for the short time period of the measured data in the ramp loading test. The experiment verifies the feasibility of the method of ramp-constant strain history.

Improvements in Brazed-Joint Properties of Silicon Nitride and Titanium Alloys Using Laser-Induced Microscale Rice Leaf Structures.

Si3N4 ceramics with a microscale rice leaf structure (MRLS) and titanium alloy were connected via brazing, and the influence of the surface microstructure on the ceramic connection was analyzed. MRLS fabrication is an efficient and high-degree-of-freedom method that can be used to change a material’s surface morphology and wettability. The MRLS was obtained at a laser power of 110 W, with line spacings of 100 and 50 μm. The laser-treated surface included nanoparticles and micro particles, exhibiting a coral-like structure after agglomeration. When the MRLS was used to braze the titanium alloy, no defects were observed at the brazing interface, and the formation was excellent. Throughout the brazed joint, the MRLS remained intact and formed a strong metallurgical bond with the brazing filler metal. A finite element analysis was performed to study the cross-sectional morphology after joint fracture; from the load-time curve, it was found that the MRLS on the surface not only helped improve the mechanical occlusion and brazing area at the interface, but also helped generate compressive stress on the Si3N4 side. Crack propagation was hindered, thereby increasing the joint strength.

Fracture mechanism of AISI 1025 rod shearing in automatic multi-stage cold forging and critical shearing speed

Experimental and numerical studies on shearing processes that focused on automatic multi-stage cold forging (AMSCF) are conducted to reveal the mechanisms of the fracture phenomena that occur in the cold rod shearing process in AMSCF. First, AISI 1025 (treated for AMSCF, specifically called SWCH25F) coil-rod of radius 17.5 mm was straightened and cut by an AMSCF machine at operating speeds of 20–90 rpm to elucidate the effects of speed on the surface quality of the sheared billet. Second, a universal testing machine was used in combination with a special shearing apparatus for experimental cold rod shearing to measure the shearing load-time (or stroke) curve and examine fracture mechanisms at sufficiently low speeds. Fractographical observations showed that speed clearly affects the cold shearing of a straightened coil-rod. The surfaces of billets sheared at low speeds showed clear evidence of ductile fracture, whereas those sheared at normal speeds did not. The experimental results were used in thermoviscoplastic finite element (FE) analysis together with the flow behaviors of the material in the cold-forging temperature range to explain the brittle fracture of the ductile material during cold coil-rod shearing in AMSCF. The concept of a critical coil-rod shearing speed for acceptable surface quality is presented.

Acoustic emission characteristics of Pykrete under uniaxial compression

Pykrete is a special building material suitable for cold regions. It has attracted extensive attention in recent years owing to its high strength and toughness. However, for structures constructed using Pykrete, appropriate damage detection and evaluation methods have rarely been investigated. As with most materials, the destruction of the Pykrete structure releases energy as stress waves and produces acoustic emission (AE) signals, which help understand the damage process and evaluate the degree of damage to the Pykrete. In this study, the AE technique was employed to monitor and evaluate the damage to different types of Pykretes and matrix subjected to uniaxial compression. Combined with the load-time curve, the conventional parameters of AE signals obtained from Pykretes and matrix were analyzed first. Furthermore, the failure modes of Pykretes and the matrix were investigated via the relationship between the ratio of the rise time to the waveform amplitude and the average frequency. The b-value was also calculated using the least-squares method to evaluate the failure process of Pykretes and matrix. The results show that the AE technique can qualitatively assess the damage process of Pykretes and has the potential to detect damage to actual Pykrete structures in situ.

Determination of Tensile Strength at Crack Initiation in Dynamic Brazilian Disc Test for Concrete-like Materials

Concrete is a brittle material whose tensile strength is about one-tenth of its compressive strength. Tensile strength is a key parameter for concrete under impact loading. When a turning point occurs before peak load in the load–time curve from the dynamic Brazilian disc test, there is no basis for choosing the turning point or the peak load to calculate the tensile strength. The objective of this study is determining the crack initiation tensile stress at the turning point or the peak. The method contrasts the time duration from Digital image correlation (DIC) and the load–time curve from a split Hopkinson pressure bar (SHPB) system in order to obtain the load value when cracking first appears. The crack initiation tensile strength is less than the peak strength for concrete specimens with a turning point in the load–time curve. The crack initiation tensile strength is equal to the peak strength for concrete specimens without a turning point in the load–time curve. The proposed method is successfully applied to determine crack initiation of concrete specimens and obtain tensile strength at crack initiation of concrete specimens.

A non-sequential isothermal local-loading forming process for large-scale rib-web components

In order to solve the problem of deformation and warpage of large-scale rib-web components by the local-loading method, we propose a new non-sequential isothermal local-loading (NSILL) forming method. First, we design the forming die and then establish a finite element (FE) model of the new NSILL forming method. The reliability of the FE model is evaluated by comparing the load-time curve and warping effect. We introduce the bending angle and extension length of the billet to comprehensively evaluate its formability. Our research found that the larger top die length, the larger the bending angle and the smaller the metal extension length. In addition, we compare these two indexes of three typical loading sequences of NSILL forming, and find that sequential loading from left to right can improve bending angle, meanwhile loading successively from both sides to the middle can reduce metal extension. We therefore propose a multiple-region control non-sequential local-loading forming method. This novel method can achieve better deformation compatibility for forming LSRW components with bending angle changed from 155.11° to 179.81° and metal extension length decreased from 89.55 mm to 0 mm, and the maximum forming load was reduced with a reduction rate of 19.24%.

Self-localization of the flexural cracks of fiber reinforced concrete beams

In order to localize the crack and to sense the crack widening of concrete, an alternative method is proposed in this study. The basic element of this method consists of six electrodes. To verify the validity of this method, four-point bending test is carried out on six types of fiber reinforced concrete beams with different fiber types. The voltage between two inner electrodes is used as the indicator for crack localization and widening. Both the voltage–time curve and the load-time curve can be divided into three stages by two turning points corresponding to the uncracked stage, the cracking stage and the crack propagation stage, respectively. In addition, the relationship between crack opening displacement (COD) and voltage may be established by an exponential function. The method is experimentally proved to be highly effective given that the voltage change and crack occur simultaneously. By increasing the number of electrodes, the method proposed in this paper can be applied to sense the crack appearance and widening in concrete structure. The method can be used to localize the cracks in fiber reinforced concrete beams.

Prediction of hardness distribution during SPD process based on FEM simulations: case study of ECAP and HPT processes

3D finite element method (3D FEM) simulations of equal channel angular pressing (ECAP) and high-pressure torsion (HPT) of Al6061-T6 alloy were carried out and analyzed. 3D FEM results were correlated and compared with those obtained experimentally and theoretically through different mathematical equations. Furthermore, the hardness was estimated using the FEM strain and theoretical strain. The simulations and experimental results were in high conformity with each other. The ECAP load−displacement curves, the HPT load-time curves, and peak loads of FEM and experimental results were close to each other. FEM simulations provided clear strain distribution maps in different planes that fully explain the plastic deformation characteristics and homogeneity in the ECAP and HPT processes. FEM effective strain results have high reliability with a slight deviation from those theoretically estimated through the mathematical equations. The hardness distribution and the strain contours maps were in good agreement, confirming the quality of the FEM results. Hardness values calculated based on FEM effective strain indicate a deviation range of 0.96%–8.8% from experimental results that support the reliability of the FEM results. Microstructure results support hardness increase because of the effect of the grain refinement after ECAP and HPT processing.